Illnesses such as schizophrenia and bipolar disorder are not the reason why violent crimes are committed by mental health patients, a study showed today.

An exhaustive study which tracked more than 8,000 patients diagnosed with schizophrenia and another 3,700 identified as having bipolar disorder over three decades in Sweden found that the abuse of illegal drugs and alcohol caused mentally ill people to perpetrate crimes of murder, manslaughter and sexual violence.

Dr Seena Fazel, a clinical senior lecturer in forensic psychiatry and consultant forensic psychiatrist at the University of Oxford, said: "The relationship between violent crime and serious mental illness can be explained by alcohol and substance abuse. If you take away the substance abuse, the contribution of the illness itself is minimal."

The academic said that all over Europe patients had been reinstitutionalised because of "this view that people with mental illness are a high risk … there's a lot of stigma". He said a solution would be to tackle drug and alcohol abuse across the whole population.

Dr Fazel added: "It's probably more dangerous walking outside a pub on a late night than walking outside a hospital where patients have been released."

He said rates of violent crime among people who were mentally ill and abused substances were no different from those among other people who abused substances.

People with mental illnesses who abuse substances have violent crime rates which are six to seven times higher than the general population – as do people with no mental health issues who have similar drink or drugs problems.

Dr Fazel said data also showed that those who were mentally ill but did not abuse substances were only at "minimally increased risk" of committing violent crime.

Around 0.9% to 1% of the general population suffers from bipolar disorder while 0.4% to 0.5% have schizophrenia.

Research has shown that around 20% of people with bipolar disorder abuse alcohol and drugs compared with about 2% of the general population. Dr Fazel said that one reason for this might be that substance abuse was "genetically programmed" into patients.

"We are looking at two reasons why this figure is higher. One is whether patients attempt to self-medicate with substance abuse. The other is that there is a possibility of genetic predisposition towards substance abuse given that schizophrenia and bipolar disorder both have an element of genetic predisposition."